In the fabrication of semiconductor integrated circuit (IC) devices, various structures such as insulation layers, metallization layers, passivation layers, etc., are formed or deposited on a semi-conducting substrate. It is a known fact that the quality of the IC devices fabricated is a function of the processes in which these structures are formed or deposited. The yield of an IC fabrication process, which in turn is a function of the quality of the device fabricated and a function of the cleanliness of the manufacturing environment in which the IC devices are processed.
The ever increasing trend of miniaturization of semiconductor IC devices occurred in recent years requires more stringent control of the cleanliness in the fabrication process or the processing chamber utilized in the process. This leads to a more strict control of the maximum amount of impurities and contaminants that are allowed in a process chamber. When the dimension of the miniaturized device approaches the sub-half-micron level, a minutest amount of contaminants can significantly reduce the yield of the IC manufacturing process. For instance, the yield of the process can be drastically reduced by the presence of contaminating particles during deposition or etching of films which leads to the formation of voids, dislocations or short-circuits resulting in performance and reliability problems in the IC devices fabricated.
In recent years, contamination caused by particles or films has been reduced by the improvements made in the quality of clean rooms and by the increasing utilization of automated equipment which are designed to minimize exposure to human operators. However, even though contaminants from external sources have been reduced, various contaminating particles and films are still generated inside the process chambers during the processing of semiconductor wafers. Some possible sources of contamination that have been identified include the process gases and liquids, the interior walls of the process chambers, and the mechanical wear of the wafer handling equipment. The chances of generating contaminating particles are also increased in processing chamber that are equipped with plasma enhancement. Various chemically reacted fragments are generated from the processing gases which include ions, electrons and radicals. These fragments can combine and form negatively charged particles which may ultimately contaminate a substrate that is being processed in the chamber. Various other materials, such as polymeric films may also be coated on the process chamber walls during plasma processing. The films may dislodge and fall from the process chamber walls when subjected to mechanical and thermal stresses such that they fall onto the wafers that are being processed.
A good example for illustrating chamber wall contamination is the etcher 10 shown in FIG. 1. Etcher 10 is a plasma etching chamber that is equipped with magnetic field enhancement generated by an upper rotating magnet 12 and a lower rotating magnet 14. The plasma etcher 10 includes a housing 16 that is typically made of a non-magnetic material such as aluminum which defines a chamber 20. A substrate holder 22 which is also a cathode is connected to a radial-frequency generator 24 which is in turn connected to a gas inlet or showerhead 26. The showerhead 26 also acts as an anode. A process gas 28 is supplied to chamber 20 through the gas inlet showerhead (or the manifold plate) 26. A semi-conducting substrate 30 to be processed is positioned on the substrate holder or cathode 22.
The semi-conducting substrate 30 is normally held against the substrate holder 22 by a clamp ring 32. During a plasma etching process, a semi-conducting wafer 30 heats up during the process and therefore must be cooled by a cooling gas from a cooling gas supply (not shown) such that heat can be transferred to a water cooled wafer holder 36. The function of the clamp ring 32 is also to hold the wafer 30 down against the pressure generated by the cooling gas. An exhaust port 34 which is connected to a vacuum pump (not shown) evacuates the chamber. During an etching process, the upper rotating magnet 12 and the lower rotating magnet 14 function together to provide a magnetic field inside the process chamber 20.
In a conventional cleaning process for a plasma etch chamber 10, a cleaning gas supply is first flown through the gas inlet port 26 into the chamber 20 and then, the RF generator 24 is turned on. This cleaning procedure is conducted after a predetermined number, e.g., between 100.about.500 wafers, have been processed in chamber 20. A plasma of the cleaning gas ions is formed in the space between the showerhead 26 and the wafer holder 32 to loosen the contaminating particles and films from the chamber walls and the upper electrode or showerhead 26.
In a silicon nitride etching process, it was discovered that when a wafer is coated with a photoresist layer, patterned and then placed in an etch chamber for a plasma etching process, the etchant gas flown into the chamber also etches the photoresist layer which is normally of a polymeric base. A byproduct of the photoresist layer when etched by the etchant gas is a fluorine-containing or Teflon-type polymer which deposits on the chamber walls and the upper electrode. When the polymeric contaminating film deposited on the chamber walls become too thick, the gravity and the mechanical stress generated by the pressure differential each time the chamber door is opened for loading or unloading loosen the polymeric films from the chamber walls. These contaminating films or particles dislodge from the chamber walls during a nitride film etching process onto the wafer surface. The contamination decreases the yield significantly since nitride etching is frequently used to define the activation regions in an IC chip.
FIGS. 2A.about.2D provides an illustration of the effect of such contamination. In FIG. 2A, a pre-processed semi-conducting substrate 40 which is coated with a silicon nitride layer 42 is provided. On top of the nitride layer 42, a photoresist layer 44 is deposited and patterned. During a subsequent etching process for the silicon nitride layer 42, a polymeric film 46 dislodged from the chamber walls falls on the surface of the wafer 40. The polymeric film 46 acts as a mask during the nitride etching process and as a consequence, the area covered by the polymeric film is not etched away during the nitride etching. This is shown in FIG. 2B.
In a subsequent processing step where a field oxide isolation (FOX) is grown, FOX does not grow in the area that is still covered by the silicon nitride. Therefore, only a very small field oxide 50 is grown. This is shown in FIG. 2C. For comparison, FIG. 2D illustrates a normally grown field oxide 52 without the polymeric film contamination problem. The problem in growing field oxide regions (or the lack of growth of a FOX region) is frequently known as a "FOX missing" defect.
The conventional chamber cleaning method which is carried out only after a large number of wafers have been processed in the chamber has been proven to be ineffective since many defective wafers can be produced in-between the two chamber cleaning procedures. Only a limited number of defect-free wafers can be produced after each chamber cleaning procedure is carried out.
It is therefore an object of the present invention to provide a method for in-situ cleaning of an etch chamber that does not have the drawbacks and shortcomings of the conventional cleaning methods.
It is another object of the present invention to provide a method for in-situ cleaning an etch chamber that can be integrated into the process recipe as part of the etching process.
It is a further object of the present invention to provide a method for in-situ cleaning an etch chamber capable of ensuring that each etching process is carried out in a clean chamber.
It is still another object of the present invention to provide a method for in-situ cleaning a RIE chamber after an etching process is conducted on a silicon nitride film deposited on a semi-conducting substrate.
It is another further object of the present invention to provide a method for in-situ cleaning of a RIE chamber by integrating a chlorine gas cleaning step into the process recipe.
It is yet another object of the present invention to provide a method for in-situ cleaning a RIE chamber which is used for silicon nitride etching process by a main etching step and an over etching step by incorporated a chlorine cleaning procedure as the third step of the etching process.
It is still another further object of the present invention to provide a method for in-situ cleaning a RIE chamber after a nitride film etching process is conducted in the chamber by integrating a chlorine cleaning step with the main etching step for the nitride film.